1. Field of the Invention
The present invention relates to an improvement to the discharge circuit for a pulsed laser, for effecting pulsed laser oscillation by pulsed discharge at a prescribed repetition cycle so as to excite a laser medium, wherein variations in laser output caused by overshoot current at the time of discharge are eliminated.
Also, the present invention relates to a pulsed power source apparatus in which a pulse generating circuit using a power semiconductor switch is combined with a magnetic pulse compressor circuit, for generating pulses of large current with narrow width at a rapid repetition rate, and more particularly, to a pulsed power source apparatus which eliminates unstable load operations caused by energy not consumed by a load when pulsed current is supplied to the load, and which ensures magnetic resetting of the pulse transformer.
2. Description of the Related Art
xe2x80x9cTEA laserxe2x80x9d refers to a laser generated by a method in which laser oscillation is effected by applying electrical discharge to a gas at a pressure of one atmosphere or higher. In the TEA laser, a uniform glow discharge is generated between a pair of mutually opposite main discharge electrodes to form an inverted population area necessary for laser oscillation. In order to attain a glow discharge spread throughout the entire discharge space, it is necessary to perform preionization before the start of the main discharge and to ionize the entire main discharge space in advance. For an excimer laser in particular, it is necessary to have as much ionization as possible directly before the main discharge, because of the short life of electrons in the ionized gas.
Currently, such preionization is achieved by various methods such as using X-rays, spark discharge, corona discharge and the like. OF these methods, a method using corona discharge has been widely used because it is relatively simple and does not greatly contaminate the laser gas.
FIG. 10 shows an equivalent circuit of a conventional capacitor-transfer type magnetic pulse compression discharge apparatus for effecting preionization using corona discharge.
In the discharge circuit in FIG. 10, a corona preionization electrode 4 is placed at a side of a main discharge space 3 formed between a pair of main electrodes 1, 2. Preionization of the laser medium in this main discharge space between the main electrodes 1, 2 is brought about with UV light generated by corona discharge by the corona preionization electrode 4.
In this configuration, a capacitor C0, connected to a high voltage power source HV, is charged with electrical charge from the power source HV.
Next, when a switching circuit SW, comprising a thyratron, GTO or the like, is turned ON, current I00 flows into a magnetic switch SI1, comprising the capacitor C0, switching circuit SW, capacitor C1 and saturable reactor. After that, when the voltage in the capacitor C1 rises to a prescribed voltage, the magnetic switch SI2 becomes saturated and enters a low impedance state.
As a result, current I01 flows through a loop formed by the capacitor C1, main discharge capacitor Cp and magnetic switch SI2, and the voltage in the capacitors Cp, Cb rises.
Afterwards, the voltage at the corona preionization electrode 4 rises by means of the preionization capacitor Cb to a prescribed voltage at which preionization starts. At that time, corona discharge is generated in the corona preionization electrode, current I02 flows through the main discharge space 3, and preionization occurs in the main discharge space 3.
Afterwards, the voltage in the main discharge capacitor Cp rises as charging takes place. When this voltage reaches a prescribed voltage at which the main discharge begins, the main discharge starts between the main electrodes 1, 2 and current I03 flows. Then, the laser medium is excited by the main discharge generated between main electrodes 1, 2 and laser light is generated.
Then, the voltage of the capacitors Cp, Cb rapidly decreases as a result of the main discharge and, after a prescribed period of time, returns to the state before charging started.
Pulsed laser oscillation results from repeating such a discharge operation at a prescribed repetition cycle (pulse oscillation frequency) established in the switching circuit SW.
FIG. 11 shows the waveform of the voltage VD applied between the main electrodes 1, 2 for the cycle of one pulse.
As discussed above, the voltage VD increases (in this drawing, becomes more negative) with the charging of the main capacitor Cp. When this voltage VD reaches the prescribed voltage at which the main discharge starts, the main discharge is generated. The voltage VD rapidly drops after the main discharge is generated and at this time, overshoot voltage, with a polarity opposite to that of the discharge voltage, is generated due to a transient phenomenon. Then, directly before the return to a steady state, a voltage Vd (shaded portion) is generated. This voltage Vd is thought to be a voltage reflected from the power source HV of the aforementioned overshoot voltage.
In other words, because the magnetic switches SI1, SI2 are in a low impedance state after discharge, the overshoot current generated directly after discharge passes through magnetic switches SI1, SI2 and flows to the power source HV, where the reflected voltage Vd is generated as a result of the reflected reverse current Id (dashed line in FIG. 10) flowing into the main discharge capacitor Cp. The main discharge becomes unstable due to this reflected voltage Vd and causes variations in laser output.
Further, a part Idxe2x80x2 of the reverse current Id flows into the preionization capacitor Cb. As a result, the preionization discharge becomes unstable, causing the preionization to become unstable and resulting in variations in laser output.
The phenomenon of the reflected voltage Vd is explained in more detail referring to FIGS. 12-14.
FIG. 12 shows an example of a conventional pulsed power source apparatus. In FIG. 12, a first-stage capacitor C0 for power is provided in a pulse generating circuit 21. This capacitor C0 is initially charged by means of a high voltage charger 22 and, as a semiconductor switch SW is turned on, supplies pulsed current I0 from the capacitor C0, via a reactor L0, to a pulse transformer PT.
A magnetic reset circuit MR1 prevents magnetic saturation of the iron core of the pulse transformed PT by supplying DC bias current to the reset coil of the pulse transformer PT.
Two magnetic pulse compressor circuits 231, 232 are connected in a cascade arrangement on the secondary side of the pulse transformer PT. In the first magnetic pulse compressor circuit 231, the pulsed current I1 whose voltage is raised by the pulse transformer PT, effects high voltage charging of the capacitor C1. This charged voltage in the capacitor C1 actuates a saturable reactor SI1 operating as a magnetic switch so that a narrow-width pulsed current I2, having undergone magnetic pulse compression, is supplied in the polarity shown in the drawing to the next magnetic pulse compressor circuit 232. In the same way, as a result of the saturable reactor S12 operating a magnetic switch, the magnetic pulse compression of the pulse width is carried out by the magnetic pulse compressor circuit 232 and the pulsed current I3 is output with the polarity shown in the drawing.
Meanwhile, magnetic reset coils and magnetic reset circuits MR2 and MR3 are provided in saturable reactors SI1 and SI2, respectively. These are excited and saturated with the reverse polarity by the supply of a direct current after saturation of the saturable reactors SI1, SI2.
The pulse output of the magnetic pulse compressor circuit 232 supplies narrow width, high voltage pulsed current to a load 24 such as a laser head chamber. In the load 24, a peaking capacitor Cp is provided in parallel with the circuit of main discharge electrodes 1, 2 and preionization electrode 4. When the peaking capacitor Cp is charged to a certain voltage level with the pulsed current, gas in the laser tube is preionized with the discharge of the preionization electrode 4 via the capacitor Cb and the main discharge between the main discharge electrodes 1, 2 is attained with this preionization.
The aforementioned configuration shows the case where the magnetic pulse compressor circuits are provided in two stages. However, N-stage structure is also possible. FIG. 13 shows the waveform of the charging voltage VCO-VCN, Vcp of the capacitors C0, C1-CN and the peaking capacitor Cp in an N stage structure. Due to magnetic pulse compression, the charging time T1-Tp undergoes greater magnetic pulse as the stage of capacitors is later so that a narrow-width discharge current is supplied to the main discharge electrodes 1, 2 of the load 24.
In a pulsed power source apparatus with such a structure, the discharge in the load 24 does not consume all the pulse energy provided. Instead, part of the unconsumed energy is returned to the pulse generating circuit 21. This returned energy is called xe2x80x9ckickback energyxe2x80x9d. This kickback energy appears as a reflected energy from the pulse generating circuit 21, in the form of recharge voltage (residual charge) for the peaking capacitor Cp after the discharge in the load.
In regards to the voltage waveform of the peaking capacitor Cp, the amount of recharge voltage varies widely depending on the state of the gas filled in the discharge tube at the time of discharge in the load 24. Therefore, when the load 24 is a laser head, the output energy may become unstable.
FIG. 14 shows an example of the voltage waveform in the peaking capacitor Cp. In this figure, the peaking capacitor Cp is discharged rapidly through the main electrodes 1, 2 following the charging period (t0xe2x88x92t1). Then, the capacitor Cp is recharged by the kickback energy during the recovery period (t2xe2x88x92t3) for recovery from discharge by the main electrodes 1, 2. This residual charged energy, which changes according to the state of gas in the discharge tube of load 24, is supplied again to the pulse generating circuit 21.
During the recovery of the discharge in the load, the voltage change of the peaking capacitor Cp takes the form of a waveform A1 or waveform B1 in FIG. 14. The waveform A1 shows the case where voltage of the peaking capacitor Cp quickly recovers to the initial state while Its positive polarity being unchanged. The waveform B1 shows the case where the peaking capacitor Cp is recharged in reverse polarity and recovers to the initial state with delay.
In the recovery process of the waveform B1, the state within the chamber where the main discharge electrodes 1, 2 and the preionization electrode 4 are installed, are influenced so that, in the case where the load is the laser head, the operation of the load becomes unstable, causing such phenomenon that the output energy of the subsequent discharge becomes unstable.
In view of the foregoing, it is an object of the present invention to provide a discharge circuit for pulsed laser which can attain stable laser output, without negative effects caused by overshoot voltage directly after the main discharge in the discharge circuit.
It is another object of the present invention to provide a pulsed power source apparatus which prevents unstable operation of the load due to residual charge in the peaking capacitor and prevents magnetic deflection of the pulse transformer.
To achieve the above objects, the present invention provides a discharge circuit for pulsed laser comprising a power source; main discharge electrodes for generating a laser beam; a main discharge capacitor charged with electrical charges for generating the main discharge between the main discharge electrodes; and a switching circuit for performing switching operations to charge the main discharge capacitor with electrical charges supplied from the power source in a prescribed repetition cycle, characterized in that a circuit element for consuming or grounding the reverse current from the power source caused by overshoot generated directly after the main discharge, is provided parallel to the main discharge capacitor.
With such a configuration as described above, a resistor or a one-way circuit element such as a diode is connected parallel to the main discharge capacitor. Such a diode or resistor allows the reverse current from the power source, caused by the overshoot generated directly after discharge, to be grounded or to be consumed as heat. As a result, it is possible to eliminate the reflected voltage from the power source which is a reflection of the overshoot voltage. Thus, stable laser output can be attained.
Further, the present invention provides a discharge circuit for pulsed laser comprising a power source; main discharge electrodes for generating a laser beam; a main discharge capacitor charged with electrical charges for generating main discharge between the main discharge electrodes; a preionization electrode for preionizing a space between the main discharge electrodes; a preliminary discharge capacitor charged with electrical charges for generating the preliminary discharge at the preionization electrode; and a switching circuit for performing switching operations to charge the main discharge capacitor and preionization capacitor with electrical charges from the power source in a prescribed repetition cycle, characterized in that a one-way circuit element is provided in a direction opposite to the direction in which the reverse current flows from the power source caused by overshoot generated directly after the main discharge, and serially to the preionization electrode.
With such a configuration as described above, a one-way circuit element such as a diode, arrange in the reverse direction to the reverse current, is connected serially to the preionization electrodes. This can prevent the reverse current from flowing to the preionization electrodes; the preliminary discharge is thereby made stable and it becomes possible to attain uniform preionization.
Another aspect of the present invention provides a pulsed power source apparatus comprising a pulse generating circuit for generating pulsed current from an initially charged capacitor via a pulse transformer as controlled by the semiconductor switch and a magnetic pulse compressor circuit for performing magnetic pulse compression on the pulsed current attained on a secondary side of the pulse transformer by means of a magnetic switch operation of a saturable reactor, and supplying the pulsed current thus compressed to a load having main discharge electrodes and a peaking capacitor connected parallel to the main discharge electrodes, characterized in that a series circuit of a diode and a Zener diode is provided in a parallel connection to the peaking capacitor of the load; the diode being oriented to prevent the peaking capacitor from being recharged in the opposite polarity after being charged during discharge recovery of the main discharge electrodes; and the zener diode having Zener voltage for generating clamp voltage to prevent the secondary side of the pulse transformer from entering a short circuit state in response to the application of magnetic reset voltage for putting the pulse transformer in an unsaturated state.
In the above configuration, the diode and Zener diode series circuit is provided parallel to the peaking capacitor of the load, thereby to block the recharge voltage of the peaking capacitor with the conduction of the diode and eliminate unstable operation of the load, while ensuring magnetic reset of the pulse transformer by generating clamp voltage on the secondary side of the pulse transformer with the Zener voltage generated by the Zener diode. Moreover, the saturation and magnetic deflection of the pulse transformer can be prevented. In other words, the parallel installation of a diode circuit for preventing the peaking capacitor from being recharged with the reverse polarity after the peaking capacitor is recharged with the discharge recovery of the main discharge electrodes of the load, can eliminate the phenomenon of instability of the load due to the recharging of the peaking capacitor, and can ensure magnetic reset of the pulse transformer after discharge and prevent the magnetic deflection and saturation thereof, because it generates the clamp voltage necessary for magnetic reset of the saturable reactor of the magnetic pulse compressor circuit with the Zener diode.